It was demonstrated that a diagnostic PCR for Borrelia burgdorferi can be inhibited in the presence of more than 500 ng of host (monkey skin) DNA. The inhibitor is the host DNA itself. An acceptable analytical sensitivity can be obtained by diluting the skin/B. burgdorferi proteinase K lysate below the inhibitory concentration of host DNA. Dilution of the lysate may obviate the need for further DNA purification.